Geography 2412 Lecture Notes

Chap 5: Ecosystems

The Biosphere is divided into various types of units for detailed analysis. As we saw last Chapter, the large vegetation/climate complexes (like forests, tundra, and deserts) are called biomes. The flows of matter and energy within biotic systems are called food chains. But we have to recognize that the term "ecosystem" has various definitions, and is used by different groups flexibly:

One might speak of the Boulder Open Space as an ecosystem, or Yellowstone national park as an ecosystem, or a pond as an ecosystem. This might be sloppy at first, but if you define the elements and their inetracitons (e.g., flow of mass or energy), then you are giving the definition some power.

This chapter examines ecosystems as flows of energy and mass (food chains) and as assemblages of plant and animal species that interact in a given geographical area, controlled by biotic and abiotic limits (e.g., climate). I will stick mostly to that second idea of ecosystem, although they build on each other (e.g., the assemblages of species often interact as food chain; that is, members play specific roles and eat each other!).

Ecosystems as Food Chains (sections 6.3 and 6.3)

Ecosystems defined as systems that cycle energy and mass among species (food chains---complexes of food chains are called food webs) are defined by the roles that species play (consumers and producers) of organic energy. The system is driven by primary productivity (net photosynthesis) that is then partitioned among the members, from primary consumers on up to the top predators. You do not need to memorize the material about food chains, but do know that:

Food chains and other forms of ecosystems are shaped by limiting factors, usually emanating form the abiotic environment: like sunshine, climate (temperature and precipitation), and nutrients from the solid earth (rocks and soil). Any species has a certain ranger of tolerance to these factors (tolerance thresholds) which set that species’ range and potential population. Typically species reach their greatest potential at optimum conditions.

Terrestrial Ecosystems

We’ll focus in this class only on terrestrial ecosystems.

The climate limits mentioned above are chiefly responsible for the distribution of ecosystems on the earth’s surface, so the biomes (Fig. 6.8) reflect the climate limits mapped in Fig.6.4. Three types: no climate limits; seasonal limits; year-round limits.

Biomes are large-scale ecosystems shaped by limiting factors and defined by mixtures of major components, like forests, and climate and/or terrain manifestations, like desert and mountains. The main biomes are listed on Fig. 6.8.

Terrestrial biomes

• Forest

• Savanna

• Grassland

• Desert

• Tundra

The structure of biomass (total weight of biotic material) varies among biomes and ecosystems. Some, like tropical forests, produce more biomass per unit area than any other ecosystem and maintain sizeable biomass well above ground, while others, like grasslands and tundra, produce less (due to moisture and temperature limits, respectively) maintain a lot of biomass at ground level and in the soil.

Composition: the most common notion of an ecosystem is as an interacting assemblage of species. As such, the ecosystem is defined by those species and their characteristics. The number of species sets its species diversity or biodiversity (you can also speak of the biodiversity of a geographical area, like Boulder County that might not be thought of as an ecosystem per se). The organisms that are members of a species make up that species’ population. The geographical area occupied by that species pop is its range. Species that have larger tolerance thresholds for sets of limiting factors tend to have larger ranges. Species with overlapping ranges that interact are called a community.

[Note: some ecologist use community to refer to species that not only overlap, but depend on each other for their existence---this is mutualism, but functional communities may form due to commensalism or parasitism]

Biogeogrpahical Patterns and Processes:

Basic principle is that the harsher the environment, the fewer species but larger populations (e.g., caribou on North Slope of NA). Also, harsher environs create less biomass.

A well-known local biogeographical pattern is: Altitudinal zonation, which we went over in detail in class.

Ecosystem Processes: The text discusses two main models of ecosystem dynamics. The successional model describes the ecosystem community changing over time from pioneer species to a climax community in equilibrium with limiting factors. The disturbance model describes the more realistic situation in which external forces, like fire, storms, drought, etc. causes shifting limiting factors on short time and space scales, thus making sure that ecosystems are not uniform communities that are always in equilibrium. These two models reflect our discussion early in this class of descriptive (disturbance) and prescriptive (succession) ideas of the environment.

Disturbance processes have a natural characteristic: that frequency and intensity are inversely related (negative correlation): more frequent tend to be less intense events. This is important as we discuss human attempts to intervene in disturbance eregimes, because we affect that relationship.

Human Transformations of Ecosystems

We are in this course mostly interested in ecosystems as perturbed by human actions. The spread of human populations, agriculture, cities, etc. across the globe has altered natural ecosystems greatly. These alterations are repetitive, they may be purposeful or inadvertent, and include six major patterns:

Reduction: loss of areal coverage of an ecosystem or community

Fragmentation: (accompanies reduction): breaking ecosystems into spatially-separated units or fragments.

Substitution: replacement of one or more organisms/species with others.

Simplification: reduction in the number of specie sin an ecosystem or community. Often accompanied by substitution and reduction of the species populations. But we want to complexify the notion of simplification to include:

Structural simplification: e.g., changing forest composition to even-aged stands, which also means all trees have roughly the same height and diameter.

Process simplification: a common human intervention is in the successional cycle, where we want ecosystems to stay in one successional stage as long as possible. We may also intervene in predation and invasion processes.

A theme of this class is that these interventions have unintended consequences:

Reducing fire causes forests to grow to climax stage and then experience fewer but more intense fires

Reducing predators may cause the prey poip to increase and become less healthy.

Contamination: introduction of pollutants, typically man-made chemical constituents. Both words, contamination and pollutants, refer to chemical alterations that are considered bad for ecosystems health.

Included in this process is: Bioaccumulation/biomagnification

We also speak of modifying the concentrations of naturally-occurring compounds. This causes:

Overgrowth: this term refers to the negative manifestations of a broader human alteration of ecosystems: fertilization (though it is not described this way in the text). Humans have increased nitrogen fixation in terrestrial ecosystems by adding fertilizers and altering plant species to use the added fertility. This purposeful act allows us to grow more and larger crops, grass, and trees. We have also inadvertently fertilized ecosystems by creating a stream of waste nitrogen and other nutrients into soil and water, which has resulted in eutrophication of freshwater lakes.